This research project attempts to define a biochemical and morphological model for the mechanisms of elongation and contraction in teleost photoreceptors. Fish rods and cones elongate and contract in response to light and darkness. These photoreceptors provide a useful model system for studying the mechanism of cell shape change because they undergo cyclic, slow, and linear elongation and contraction in response to a discrete triggering activation. This project will attempt 1) to ascertain the disposition of microtubules, actin filaments, and possible myosin filaments in contracted, elongated and in intermediate states, 2) to characterize the susceptibility of elongation and contraction to conventional motility inhibitors and other agents, 3) to investigate the control mechanisms which activate elongation and contraction, and 4) to characterize the proteins present in the contractile myoids of rods and cones in order to define further the contractile elements and to search for regulatory proteins.